patch 8.0.1638: popup test fails depending on environment variable

Problem:    Popup test fails depending on environment variable.
Solution:   Reset $COLORFGBG when running Vim in a terminal. (closes #2693)

src/testdir/screendump.vim

@@ -30,6 +30,13 @@ func RunVimInTerminal(arguments, options)
   " Remove it here.
   call delete(".swp")
 
+  if exists('$COLORFGBG')
+    " Clear $COLORFGBG to avoid 'background' being set to "dark", which will
+    " only be corrected if the response to t_RB is received, which may be too
+    " late.
+    let $COLORFGBG = ''
+  endif
+
   " Make a horizontal and vertical split, so that we can get exactly the right
   " size terminal window.  Works only when we currently have one window.
   call assert_equal(1, winnr('$'))

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1638,
 /**/
     1637,
 /**/